The operation of engine control units, which are used for controlling combustion engines, requires electric power which is generated by an electric generator that is likewise driven by the combustion engine. When the engine is not running or at vehicle start-up, a vehicle battery must supply the power, which is why the engine control unit is electrically connected to a vehicle battery.
Moreover, the engine control unit must maintain specific functionalities of the vehicle electronics, even in the case of a parked vehicle. These include, inter alia, the operation of an elapsed-time meter for monitoring the time that has elapsed since the last engine operation cutoff, or the maintaining of a wake-up function by a data bus, in which case another control unit transmits a pulse via a communication line to the engine control unit, thereby switching it on. Continually supplied memories within the vehicle electronics also require a permanent connection of the engine control unit to the vehicle battery since their data can be lost when the engine control unit is switched off.
From German Patent Application No. DE 10 2004 023 252, a power supply circuit for a vehicle electronic control unit is known which has a constant-voltage power supply circuit that is supplied with power from the vehicle battery and that carries a predetermined, controlled voltage to a microprocessor of the vehicle electronic control unit, thereby continuously supplying it with power. The constant-voltage power supply circuit is connected in a switched operation via a plurality of intermediate-voltage circuits which are used for separating the various operating states of the vehicle electronic control unit, respectively of the vehicle.